major fix about slice tags

major fix about slice tags
no slice tags was exposed by sfi describe before this change
we now expose them properly with a 'scope' field that can be either 'slice' or 'sliver'
note that nodegroup-based tags are still silently ignored at this point
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using allocate with the 'append' option is probably the best approach anyway, with an empty list of slice tags; this avoid resetting slice tags